Recently many vehicles are equipped with electric power steering systems, in which steering of a vehicle is assisted by toque of a multi-phase (three-phase) electric motor driven when a steering wheel is operated. The three-phase electric motor is connected to a three-phase inverter, which includes switching elements such as transistors for selectively connecting each phase of the motor to the positive terminal and the negative terminal of a direct current (DC) power source. By controlling the three-phase inverter, torque of the motor is controlled thereby to assist steering of the vehicle.
In this electric power steering system, the controllability of the steering angle is likely to be lowered if any one of the electric connection paths such as electric cables between the motor and the inverter is disconnected. For this reason, various countermeasures are proposed.
For example, JP 8-163889 (patent document 1) proposes to check for any short-circuit or disconnection abnormality in electric connection paths based on electric currents actually flowing in a three-phase electric motor in energizing a specified phase of the motor. JP 2005-295688 (patent document 2) proposes to check for any abnormality in electric connection paths based on magnitude of currents actually flowing in a three-phase motor in controlling a three-phase inverter. Further, JP 2007-274849 (patent document 3) proposes to check for any abnormality in electric connection paths based on magnitude of currents actually flowing to a three-phase electric motor in energizing a three-phase inverter.
According to the patent document 1, if the connection path between the specified one of the three phases of the motor and the inverter is disconnected and the specified phase of the motor is also shot-circuited to another phase, this abnormality cannot be detected. According to the patent documents 2 and 3, abnormality may be erroneously detected due to variations in the resistances of the electric connection paths and temperature characteristics of the resistances.